Communication systems, as they are known in the art, include systems such as radio communication, paging, cellular telephone, and other similar types of systems. It is common for communication systems such as these to generate information signals which are redundant in nature due to the addition of error correction and detection bits, the addition of redundant information packets, and other similar techniques that help increase the probability of communication devices which are utilizing the system to receive the transmitted information. In trunked radio system specifically, a trunked central controller sends information signals in the form of signalling words to a plurality of radios which are part the system in order to coordinate their activities. The central controller will ordinarily create convolutional coded signalling words which contain error correction bits, and error detection bits. The central controller also duplicates the signalling words in order to increase the chances of the radios in the system receiving the information signals. This is especially helpful in radio communication systems which are prone to interference.
Trunked radio systems, as they are known in the art, allocate communication resources amongst users of the system. A resource controller or control system (also commonly referred to as the trunked central controller) for a trunked radio system transmits control information to trunked radios which are on the system, usually over a control resource (also known as the control channel). A trunked portable radio receiving a particular control channel detects instructions from the central controller that might, for example, instruct the radio to shift to a particular communication channel (repeater) whereupon the radio (subscriber unit) can transmit and receive voice and/or data information on the particular channel which has been assigned.
Other trunked systems, for example, may not utilize a dedicated control channel and transmit the control information over all the repeaters (or channels) available in the system.
In a trunked system, such as the SMARTNET.TM. trunked radio system manufactured by Motorola, Inc., signalling words (normally referred to as outbound signalling words hereafter referred to as OSWs) are generated by the system central controller and are then transmitted over the control channel continuously to all of the portable radios in the system. The OSWs inform specific radios or groups of radios when to move to one of the channels available on the system, and also informs the radios of other important system information. The OSWs are comprised of a string of data bits which carry system information to the radios. The portable radios on a trunked system spend a great deal of the time receiving the information from the control channel and decoding the received information in order to determine if any information is directed to the specific radio. When the system is in an idle state, all radios in the system have their receivers monitoring the control channel for information thereby remaining synchronized with the control channel.
In the SMARTNET.TM. system the outbound signalling words (OSWs) generated by the central controller have redundant data bits in the form of error correction and detection bits which approximately double the length of each of the OSW packets. The OSWs are then run through a convolutional algorithm such as is known in the art, which makes each OSW a 1/2 rate convolutional code data string. By having a 1/2 rate convolutional code, the system increases the correction of burst errors of up to 19 consecutive bits. The central controller will also normally repeat each of the OSW packets several times in order to increase the chances of reception by the portable radios. While the addition of redundant error correction/detection bits, and the repetition of the OSWs increase the chances for the radios in the system to receive the OSWs, even under noisy channel environments, they decrease the battery life of the radios since the radios need to spend a great deal of time receiving redundant information. Since under normal system conditions the radio frequency channels are normally "clean" enough for the radios to receive the OSWs without any problems, a need exists for a way of minimizing the amount of time radios spend decoding the OSWs in order to gather all the information the radios require, and thereby extending the battery life of the radios.
Communication systems which utilize a nonsynchronous coded squelch such as DIGITAL PRIVATE LINE.TM. (DPL.TM., trademark of Motorola, Inc.) also have the same problems of the communication device spending too much time decoding the incoming cyclic information. The DPL.TM. code is a (23, 12) cyclic Golay code used to provide a private line for the communication device receivers. A need for a faster decoding algorithm which can conserve battery life on these types of systems is also required in order to allow the communication devices to operate for longer periods of time.